Electroless deposition of palladium



United States Patent 3,235,392 ELECTROLESS DEPOSITION OF PALLADIUM .lohn Joseph Miles, Liverpool, England, assignor to Automatic Telephone & Electric Company Limited, Liverpool, England, a British company No Drawing. Filed Oct. 9, 1961, Ser. No. 143,577 Claims priority, application Great Britain, Oct. 11, 1960, 34,764/ 60 2 Claims. (Cl. 106-1) This invention relates to electroless plating or chemical plating, that is to say, to the deposition of metals without electrolysis, and is concerned, in particular, with the electroless deposition of palladium.

In comparison with electrolytic plating methods, the deposition of metals by electroless or chemical techniques involves the simple chemical formation of the desired metal by interaction of the components of the plating bath used,'followed by deposition of such metal upon articles immersed in the bath. Known chemical plating baths suffer from certain disadvantages. For instance, acid-type baths tend to remove zinc from brass articles upon which a deposit is to be made, which adversely affects the quality of the resultant plated coating. Alkalitype baths are unstable and are liable to sufier spontaneous decomposition.

It is an object of the present invention to provide improved chemical plating baths for the deposition of palladium. A more specific object of the invention is to provide novel palladium compounds for use in such baths and also to provide baths which give adherent smooth bright deposits on a variety of metallic supports or basis metals and which avoid dezincification of brass articles.

According to the invention, a chemical plating bath for the electroless plating of palladium comprises an aqueous solution containing a source of palladium ions in complexed form and a source of sulphamate ions, the aqueous solution having a pH in the range of 1.0 to 3.5.

Preferably, the palladium is present in the plating bath as a sulphamate complex thereof.

The proposed bath is essentially an acid solution of palladium complexed as sulphamate, sulphate, diamminosulphamate, diammino-sulphate, dinitrito-disulphamato or other palladium complexes, which deposits palladium on to suitable metallic surfaces by simple immersion. The preferred concentration of the metal in the bath is 1 to 100 g./l.

It is essential that there be free sulphamic ions present, the concentration being such as to maintain the pH in the desired range.

One of the advantages of the proposed bath is that, on brass alloys, dezincification is negligible and the quality of the coating is thus improved.

The deposits are adherent, smooth and bright and atlord a high degree of protection against normal atmospheric corrosion. Satisfactory deposits can be obtained on copper, copper alloys, nickel, silver, germanium, steel and aluminium.

A typical bath in gms. per litre consists of:

Palladium (as metal) Palladium sulphamate 30 Sulphamic acid, about 10 For copper and its alloys, a pH range of 1.5 to 2.5 is suitable, that is, a pH in the lower portion of the aforementioned overall range of 1.0 to 3.5; for less reactive metals than copper and its alloys, such as nickel and silver, an even lower pH range of 1.0 to 2.0 is suitable; for more reactive metals such as aluminium and steel, a pH range of 2.5 to 3.5 is preferred, namely, in the upper portion of the overall range.

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The solution is contained in non-reactive vessels such as glass or polyethylene. Since the rate of deposition is a function of temperature, the solution may require to be heated.

Typical rates of deposition on copper using the above solution are:

Inch palladium per hr. At 20 C. 0.00003 At C. 0.00016 The articles to be plated are cleaned in a suitable manner, i.e. as for conventional electroplating, and preferably rinsed in deionized water prior to immersion in the plating bath.

Deposition takes place immediately on immersion and ceases on withdrawal. This means that the solution is stable and, provided dust and foreign matter are excluded, no loss of palladium is experienced from spontaneous decomposition.

Thin, protective deposits of palladium are readily obtainable on printed circuits, connector springs, plug and socket components. Silver and silver-plated articles can be given a cheap non-tarnishable finish. Germanium can be readily given a deposit of palladium to facilitate electrical connections thereto.

In order that the invention may be readily understood, the following specific examples are given by way of illustration.

Example I 25 gms. of palladium diammino-dinitrite salt were added slowly, i.e. about 5 gms. at a time, to ml. of an aqueous solution containing 20 gms. sulphamic acid at about 50 C. A vigorous reaction occurred with evolution of nitrogen. When dissolution was complete, the solution was brought rapidly to the boil and then cooled quickly to 50 C. and maintained at that temperature for 4 hours, taking care to prevent loss of volume by evaporation. The volume was adjusted to 1 litre, which contained 10 gms. of metallic palladium as the diamminodisulphamate.

A piece of copper was cleaned in the conventional manner for subsequent electroplating and immersed in the solution prepared as above. Deposition of palladium took place immediately by ionic replacement, which process gradually becomes supplanted by What appears to be auto-catalytic deposition of palladium, which proceeds indefinitely or until the piece is withdrawn.

At room temperature, a thickness of 0.00001 inch of palladium is obtained in 15 minutes, this being largely achieved by replacement. In 60 minutes a thickness of 0.00003 inch is obtained, the increase in thickness being due to auto-catalytic reduction. The coatings are smooth, bright and adherent.

Example 2 13 gms. of palladous hydroxide, prepared as described in co-pending application Serial No. 204,627, filed June 22, 1962, were added slowly, in 5 gm. portions, to 100 mls. of an aqueous solution containing 20 gms. of sulphamic acid at about 50 C. The solution was treated as described in Example 1 and used for the electroless plating of copper, with substantially the same results.

Example 3 A solution containing 20 gms. palladium per litre was prepared as described in Example 1, the ratio of sulphamic acid to metallic palladium content being maintained in the ratio of 2 to 1.

After suitable cleaning, a piece of copper alloy was immersed in this solution for 60 mins. at a temperature 3 of 50 C. A smooth, adherent coating of palladium about 0.00006 inch was obtained.

Example 4 A solution containing 30 gms. palladium per litre was prepared as described in Example 1, the ratio of sulphamic acid to metallic palladium content being maintained in the ratio of 2 to 1.

A piece of copper alloy, immersed for 60 mins. after suitable cleaning, gave the following thicknesses of palladium as a smooth, adherent coating:

Inch

At 20 C. 0.00012 At 50 C. 0.0002

At 75 C. 0.0004

I claim:

sulphamic acid in a concentration sufficient to maintain the pH of the aqueous solution in the range of 1.0 to 3.5.

2. A method of electrolessly plating palladium onto an article having a metal surface, which consists essentially in at least partially immersing an article having a metal surface in a chemical plating bath consisting of an aqueous solution containing (1). a source of ions of palladium in complexed form in an amount of from 1 to 100 g./l. of palladium as metal, said source being selected from the group consisting of palladium diamminodisulphamate, palladium diammino-sulphate, ammonium disulphamato-dinitrito-palladite, ammonium sulphate-dinitrito-palladite, disulphamato-dinitrito-palladous acid and sulphato-dinitrito-palladous acid and (2) sulphamic acid in a concentration sufficient to maintain the pH of the aqueous solution in the range of 1.0 to 3.5.

References Cited by the Examiner UNITED STATES PATENTS 2,361,203 10/1944 Holdaway et al. 20446 XR 2,458,839 1/1949 Dyer 117130 2,720,494 10/1955 Suter et al. 20447 2,984,604 5/1961 Duva et al. 20447 ALEXANDER H. BRODMERKEL, Primary Examiner.

JOSEPH REBOLD, MORRIS LIEBMAN, Examiners. 

1. A CHEMICAL PLATING BATH FOR THE ELECTROLESS PLATING OF PALLADIUM ON A METAL SURFACE, WHICH CONSISTS ESSENTIALLY OF AN AQUEOUS SOLUTION CONTAINING (1) A SOURCE OF IONS OF PALLADIUM IN COMPLEXED FORM IN AN AMOUNT OF FROM 1 TO 100 G./L. OF PALLADIUM AS METAL, SAID SOURCE BEING SELECTED FROM THE GROUP CONSISTING OF PALLADIUM DIAMMINO-DISULPHAMATE, PALLADIUM DIAMMINO-SULPHATE, AMMONIUM DISULPHAMATO-DINITRITO-PALLADITE, SMMONIUM SULPHATO-DINITRITO-PALLADITE, DISULPHAMATO-DINITRO-PALLADOUS ACID AND SULPHATO-DINITRITO-PALLADOUS ACID AND (2) SULPHAMIC ACID IN A CONCENTRATION SUFFICIENT TO MAINTAIN THE PH OF THE AQUEOUS SOLUTION IN THE RANGE OF 1.0 TO 3.5. 